The invention relates to a method for image inspection and color guidance for printing products of a printing press, in which preferably in on-line operation, actual image data of the printed images of the printed products are ascertained and compared with nominal image data for the purposes of locating defects.
A method of this type is disclosed in U.S. Pat. No. 5,187,376. By the method, in the ongoing production run mode, the printed images of the continuously produced printed products are examined for defects by means of an optical detection zone. This is done by acquiring actual image data from the printed images on the printed products and comparing them with nominal image data. The nominal image data originate from a defect-free original image, so that in the aforementioned comparison, if a deviation between the actual and nominal data occur, the conclusion can be drawn that there is a defect. While a deviation that occurs does indicate the presence of a defect, nevertheless in principle it is not possible to learn what type of defect is involved. If various available provisions were made for eliminating the defect without knowing the precise type of defect, this can cause problems in the production run.
It is also known that on the occurrence of a defect, defective sheets of a sheet-fed printing press are separated; this can be done for instance by means of a by-pass or by inserting seperators. Although these known provisions do accomplish defect designation, nevertheless they do not include defect analysis. With these basic problems in view, it is the object of the invention to create a method for image inspection and color guidance in printed products of a printing press, in which targeted ascertainment and elimination of defects is done that is reliable in terms of the production run process.
According to the invention, this object is attained in that upon the appearance of a defect, before any change in the color guidance including ink control is made, a check is made as to whether on the basis of the type of defect, some other cause than the color guidance for the deviation is possible. That is, if as a result of the nominal/actual comparison a deviation in the image data that indicates a defect is found, then a corrective provision is done in a purposeful way, such that before a change in color guidance and in particular before the performance of color regulation, it is possible to preclude with high probability that the ascertained deviation has some other cause than the color guidance. Other possible sources of defects are all the known printing defects, such as blips, register, color, moisture, stencilling, scumming, shifting, doubling, transfer register marks, and so forth. Operation errors, such as a change of paper type, or too little ink in the ink well and also equipment defects, such as soiled optics, breakage of the optical fibers, a defective lamp, or a voltage drop in electrical devices may also be responsible for the ascertained deviation between the nominal and actual values. In all these cases, the color must not be regulated; instead, a defect report may be sent to the printer, for instance, or the corresponding defect is eliminated automatically by suitable provisions. If a color deviation is detected upon the nominal/actual comparison, then naturally the cause may be incorrect color guidance (incorrect setting of the zone opening of the inking unit), but the other defects mentioned above cannot be eliminated by adjusting the zone opening or openings. If the color control now attempts to compensate for this printing disruption by changing the zone opening, the effects are unpredictable; they can range up to instability of the control operation, yet without eliminating any defect. In the method of the invention, in the on-line image inspection before any possibly necessary correction of color guidance is made assurance is gained that all the other possible causes of the problem can be precluded. Not until it is assured that no other possible causes may be present is the color control activatedxe2x80x94lastxe2x80x94for instance by ink zone adjustment.
In a further feature of the invention it is provided that an analysis of defect type is performed, in which different predetermined criteria are employed successively, preferably systematically, for detecting the type of defect. Thus for each type of defect, certain criteria are specified, which are employedxe2x80x94for a systematic search for defectsxe2x80x94so that the type of defect involved can be predicted with high probability. If the type of defect is known, then a remedy to it can be definitively carried out.
It is also advantageous that a defect elimination provision is carried out in accordance with the ascertained type of defect. Thus if a high probability implies for a certain defect, then to eliminate it only the intended concrete step need be taken.
If the defect type determination is ambiguous, the type of defect that has the highest probability is assumed to be present. Thus if a plurality of the aforementioned criteria are met in the determination of the type of defect, and if this makes unambiguous determination of the type of defect possibly more difficult, then combinations of criteria should be used. Certain combinations of the criteria allow conclusions to be drawn as to the type of defect, and additional aids, such as probability calculation, can be used in order to gain the maximum possible target accuracy in determining the type of defect.
Finally, it is advantageous if where there are a plurality of possible types of defect, a change in ink guidance, in particular activation of ink control is performed last in order to eliminate the defect. Thus not only when there is an unequivocal defect determination if an intervention into the ink control made as the last step, but it also forms the last operation if on the basis of the defect type analysis ambiguity cannot be precluded.
In accordance with the invention, there is provided a method for image inspection and ink guidance for printing products of a printing press, in which in on-line operation, actual image data of the printed images of the printed products are obtained and compared with nominal image data for the purposes of locating image defects, which comprises the steps of checking, upon the appearance of a defect, before any change in the ink feed is made, as to whether on the basis of the type of defect, some other cause than the color guidance for the deviation is possible.
According to a further feature, the method includes the step of performing an analysis of defect type, in which different predetermined criteria are performed successively, and systematically, for detecting the type of defect.
According to still another feature, which includes carrying out the defect elimination process in accordance with the analysis of the type of defect.
According to a further feature, if the defect type analysis is ambiguous, making the assumption that the type of defect that has the highest probability is present.
According to a still further feature, which includes in case the analysis indicates a plurality of possible types of defect, performing a change in ink feed, in particular activation of ink control last in order to eliminate the defect.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for image inspection and color guidance for printing products of a printing press, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
The drawings illustrate the invention, wherein:
FIG. 1 is a flowchart of the method steps on which the method is based;
FIG. 2 is a defect analysis matrix, and
FIG. 3 is a block diagram of a printing machine suitable for performing the method.